Sanitary temperature sensor calibration

Sanitary temperature sensors are commonly used in many industries, but their calibration is different and more difficult than for normal temperature sensors. T emperature is one of the critical process parameters in many industries and the accuracy of the temperature measurement in processes is a crucial consideration. The food and beverage, dairy, pharmaceuticals and life science industries have additional requirements for temperature measurement sensors because of their processes. They require temperature sensors that are sanitary, meaning that these sensors need to be suitable to be installed in hygienic and aseptic process environments. These sensors need to be hygienic and designed to be easy to clean, often supporting clean-in-place (CIP) processes. The mechanical design needs to be free from any cavities, dead pockets, gaps or anything that would complicate the hygienic cleaning. Surface finishes of these sensors are hygienically graded and need to meet the strict standards in these industries, such as the 3-AR (https://www.3-a.org/) or EHEDG (European Hygienic Engineering & Design Group) https://www.ehedg.org/. The material of the wetted parts in these sensors is often high-grade stainless steel, suitable for these applications, and for easy installation, the sanitary sensors are often provided with the clamp installation.

The temperature ranges typically go up to around 150°C, or in some cases up to 200°C, which in itself is not very challenging.

The role of calibration

In any industry, it is vital that the process measurements measure correctly and as accurately as designed. This can be achieved with the help of suitable process instruments and with a proper calibration program.

Within the food and beverage, pharmaceutical and life science industries, calibration plays an even more important role than most other industries. In these industries, the consequences of a bad or a failed calibration can have a dramatic effect, as we are talking about consumer and patient health and safety. As failed calibration can be very costly in these industries, it has to be avoided by all means possible.

These industries also have dedicated strict regulations concerning calibration.

Why are sanitary sensors difficult to calibrate?

than 100 mm and typically around 50 mm, but can also be as short as 25 mm. The diameter of the sensor typically is 3 mm or 6 mm.

The commonly used practice in temperature calibration (and a Euramet guideline recommendation) is that a temperature sensor should be immersed deep enough in a temperature bath to achieve sufficient accuracy. The recommendation is to immerse to a depth that is 15 times the sensor diameter (plus the length of the sensor element). But with these short sanitary sensors, it is simply impossible to immerse the sensor to sufficient depth during the calibration. For example, a typical sanitary sensor with a diameter of 6 mm should be immersed to at least 90 mm (15 x 6 mm) depth during the calibration to ensure accurate results. But if that 6 mm sensor has a length of only 50 mm, sufficient immersion is simply not possible.

Some rules of thumb for the immersion depth when calibrating in a liquid bath are: • 1% accuracy: immerse five diameters plus the length of the actual sensing element inside the sensor • 0.01% accuracy: immerse 10 diameters plus the length of the sensing element • 0.0001% accuracy: immerse 15 diameters plus the length of the sensing element

Figure 1: Error versus immersion. Figure 2: Calibrating normal temperature sensors versus a sanitary sensor with a flange.

The accuracy in the above rule is to be calculated from the temperature difference between the block temperature and the ambient temperature.

For example, if the ambient temperature is 20°C and the block temperature is 120°C, there is a 100°C difference. If you then immerse the probe only five times the dimension (plus the sensing element length) — say you have a 6 mm probe with a 10 mm sensing element inside of it and you immerse it 40 mm (5 x diameter + sensing element) — you can expect about 1°C error due to the low immersion (1% of 100°C).

Figure 1 illustrates the relationship between thermometer immersion depth (in diameters) and the relative error of the temperature difference (of the temperature block and ambient temperatures). So if you don’t immerse at all, you naturally get a 100% error, and if you immerse deep enough the error caused by immersion becomes insignificant.

This rule of thumb can become quite significant at higher temperatures or for extremely short sensor lengths, and so this should be kept in mind with sensors less than 40 mm.

It is not always easy to know the length of the actual sensing element inside the probe. If that is not mentioned in the datasheet, you can ask the manufacturer. Sanitary sensors often have a clamp connection with a flange As mentioned previously, sanitary sensors also often have a socalled clamp connection (Tri-clamp, ISO 2852, DIN 11851, DIN 32676, BS 4825, Varivent, etc), so there is a relatively large metallic flange that causes heat to conduct away and leak from the sensor to the flange. In practice this means that the flange causes the sensor to read a slightly lower temperature when calibrating to a temperature higher than ambient temperature. Liquid bath or a dry block?

Generally you can calibrate temperature sensors in a liquid bath or in a dry block. Pros and cons of a liquid bath A liquid bath makes it easier to insert any shape of sensors and you can also use a reference probe inserted at the same time. Depending on the size of the liquid bath, you may be able to insert several sensors to be calibrated at the same time. In case the sensor to be calibrated is an odd shape, a benefit is that it will still fit inside the liquid bath. A liquid bath often enables better uniformity and accuracy than a dry block due to better heat transfer via the liquid.

Even though this sounds like a favourable option, a liquid bath also has several drawbacks: • A liquid bath always includes some sort of liquid, such as silicone oil, and often you don’t want to contaminate the sanitary sensor in such a liquid. Cleaning is necessary after the calibration to ensure that the sensor is clean when installed back into the process. • Handling of hot oil is dangerous and any spills may cause injuries. • Liquid baths are very slow. Even fitting several sensors in at the same time, it is often several times slower than a dry block, so overall effectiveness is not really any better. Sometimes several baths are used, each set to a different temperature, but this is a very expensive way to calibrate. • The sanitary sensor should be placed so that the surface of the liquid touches the bottom of the flange, but in practice this is not always that easy to do. For example, silicone oil has a large thermal expansion, which means that the surface level is changing slightly as the temperature changes. So, you may need to adjust the height of the sanitary sensor during the calibration. • Liquid baths are often large, heavy and expensive pieces of equipment. Pros and cons of a dry block The main pros of calibrating the sanitary sensor in a dry block are: • Because it is a dry, it is also clean and does not contaminate the sanitary sensor to be calibrated. • A dry block is very fast to change temperature. • When using a dedicated insert with proper drillings, it is easy to insert the sanitary sensor the same way each time, and the calibration is repeatable every time and with different users. • A dry block is light and easy to carry compared to a liquid bath. • Typically, a dry block is also cheaper than a liquid bath.

On the downside, a dry block is a less accurate than a liquid bath, which typically only calibrates one sanitary sensor at a time and needs different inserts drilled for different diameter sensors.

Figure 3: The internal reference sensor is usually too low to be accurate with sanitary sensors. Figure 4: Using a dedicated short reference sensor.

Despite these downsides, customers often prefer to make the calibration of their short sanitary sensors in a dry block.

How to calibrate in a temperature dry block

To calibrate these short sanitary sensors in a temperature dry block, there are a few considerations to take into account. Using a reference sensor Firstly, when you do the calibration in a dry block, the flange of the sanitary sensor makes it impossible to use a normal external reference sensor in the same insert.

Comparing the calibration of a normal (long, no flange) temperature sensor using a reference probe with a short sanitary sensor with a flange (Figure 2), we can see that the short sensor flange covers all the holes in the insert, so it is not possible to insert a normal reference temperature probe. Using an internal reference sensor The dry block always includes an internal reference sensor. Trying to use the internal reference sensor in the dry block does not work with short sensors, because the internal reference sensor is located close to the bottom of the temperature block, and the short sensor to be calibrated is located in the very top part of the insert (Figure 3). Dry blocks typically control the temperature gradient on a limited range in the bottom of the insert. The top part of the insert typically has a larger temperature gradient, so the top of the insert does not have the same temperature as the bottom of the insert. The size of the gradient depends on the temperature difference between the insert and environment, and how deep you insert the sensor. Using a dedicated short reference sensor As the internal reference sensor in the bottom of the dry block is not suitable, we need to use a dedicated external reference temperature sensor. The aim is to use a dedicated reference sensor that is short enough so that it can be immersed to the same depth as the sanitary sensor to be calibrated. Optimally, the middle of the sensor elements should be aligned to the same depth.

Also, the reference sensor needs to have a thin flexible cable so that the cable can fit under the flange of the sanitary sensor. To help that, we can make a groove in the top of the insert where the reference sensor cable fits and the flange of the sanitary sensor still touches the top of the insert.

Naturally the structure of the temperature dry block needs to be such that the sanitary sensor with the flange fits in its place and touches the insert top end. In some dry-blocks the surroundings prevent the flange from going deep enough to touch the top of the insert.

Locating the reference sensor at the same depth as the short sanitary sensor to be calibrated ensures they are measuring the same temperature. Also, the reference sensor cable is in the groove, so it does not prevent the flange of the sanitary sensor from touching the top of the insert (Figure 4). Short sensor without a clamp connection There are also short temperature sensors without the clamp connection and without the flange. With these sensors you should also use an external reference sensor that has been immersed to the same depth as the sensor to be calibrated. The reference sensor should be as similar as possible to the sensor to be calibrated, with a similar diameter, similar response time, etc.

Documentation, metrological traceability and calibration uncertainty

As documentation is included in the formal definition of calibration, it is a vital part of every calibration. This is naturally also valid in sanitary temperature sensor calibration — typically in the form of a calibration certificate.

The calibration equipment used should have a valid metrological traceability to the relevant standards, otherwise the calibration does not ensure traceability in the sensor calibration.

The calibration uncertainty is also a vital part in every calibration. If the calibration equipment (and calibration method and process used) is not accurate enough for the sensor calibration, then the calibration does not make much sense.

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RFID SECURITY READERS

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SAFETY I/O MODULES

Turck has expanded its range of safety I/O components with the TBPN and TBIP block I/O modules for Profisafe and CIP Safety respectively. The IP67/IP69K modules provide safety input and output signals directly from the field to the safety controller, or they can alternatively be used as decentralised safety controllers in the field. This function optimises modular machinery and also applications in which long bus cycle times to the central controller would require greater safety distances. Safety functions can also be tested offline with the local safety controller function before the machine or the machine module communicates with a central controller. Commissioning work is considerably faster when the devices are used in combination with a custom web server. The block I/O devices provide four safety inputs and four universal inputs or outputs (FDX) in the field. The flexibility of the FDX ports in particular enables optimum coverage of the individual safety signal requirements of any application. The modules can be used for applications up to PL e, category 4, in safety circuits up to SIL CL 3. Actuators are provided with 2 A per output, with up to 9 A in total. With protection to IP65/IP67/IP69K and an extended operating temperature range from -40 to +70°C, the fully potted block modules can withstand harsh environments.

PILZ INTRODUCES THE FIRST BATCH SIZE 1 SAFETY RELAY

The myPNOZ safety relay from Pilz monitors safety functions such as E-Stop, safety gates, light curtains, two-hand controls IIIA/C and enabling switches. It consists of a head module with up to a maximum of eight expansion modules that can be freely combined.

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INDUSTRIAL DATA ACQUISITION SYSTEM

The Dewesoft IOLITEd is an industrial data acquisition system designed for easy-to-use industrial data acquisition and monitoring applications. All signal amplifiers are designed to offer high-end signal conditioning with 24-bit resolution and a high sampling rate of up to 50 kHz/channel.

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Metromatics Pty Ltd

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HEAVY-DUTY SWITCHGEAR

The Schmersal HDS switchgear series is designed for use in heavy industry. The modular series unites a range of functions into a single platform, making it suitable for a wide range of applications.

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Control Logic Pty Ltd

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VALVE ACTUATORS WITH AC POWER

The wider Rotork PAX range of linear valve actuators is made up of a variety of linear actuators and motorised regulators. PAX1 units are flexible, low-voltage actuators that are suitable for use in remote explosion-hazard locations. They offer a thrust output of up to 2890 N (650 lbf), enabling control of most regulators, small valves and pumps. They are capable of controlling pressure ranges of 0–0.5 psig to 0–3000 psig.

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Rotork Australia

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HMI/PLC DEVICES

Turck has expanded its TX700 HMI/PLC device series with two variants for food and beverage applications (TX700FB) and two variants with high brightness displays for sunlight readability (TX700HB). The FB devices are available in 7″ and 15″ screen diagonals and the HB variants in 7″ and 10″ screen diagonals.

The FB devices were developed to comply with hygienic design requirements (DIN EN1672-2, EHEDG/FDA 21 CFR 177.2006) and come with a stainless steel front with a polyester coating. The device front has been designed with protection to IP69K so that the devices are waterproof at high pressure up to 80°C. The FB products are also resistant to acids and chemicals. The HB devices with their extra bright display up to 800 cd/m2 are particularly suitable for outdoor applications. The displays are manufactured with liquid bonding (LOCA) — a screen bonding process that improves screen contrast and increases brightness through reduced reflection and refraction. The HB devices with the full metal housing are resistant to mechanical influences.

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Turck Australia Pty Ltd

www.turck.com.au

SERIAL-TOETHERNET DEVICE SERVER

The ICP DAS DS-2200i is a serial-to-ethernet device server that is designed to add Ethernet and internet connectivity to any RS-232/422/485 device and to eliminate the cable length limitation of legacy serial communication.

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The DS-2200i also adds 3000 VDC isolation and ±4 kV ESD protection that diverts the potentially damaging charge away from a sensitive circuit.

The VxComm Driver/Utility supports both 32- and 64-bit Windows 10/8/7/2008/2003/XP. The virtual COM works transparently and is protocol independent.

ICP Electronics Australia Pty Ltd

www.icp-australia.com.au